The invention generally relates to digital television signal receivers and, more particularly, the invention relates to a phase detector for a vestigial sideband (VSB) signal receiver.
To accurately extract data from a vestigial sideband (VSB) signal as used, for example, in a high definition television (HDTV) transmission system, the phase error of the VSB signal with respect to the phase of the sampling signal for the quantizer (slicer) must be detected and tracked. FIG. 1 depicts a block diagram of a conventional phase tracking loop 100 for a digital television receiver. This conventional loop is defined in the xe2x80x9cGuide To The Use Of The ATSC Digital Television Standardxe2x80x9d Document A/54, Advanced Television Systems Committee, Apr. 12, 1995. The standard suggests using a phase tracking loop that operates only upon a real signal (e.g., the in-phase signal). As such, to determine a phase error of the VSB signal, the quadrature phase (Q) signal is derived from the I signal. Because the I and Q components of a VSB signal are related by a filter function that is approximately a Hilbert transform, a filter 104 is used to derive a Q signal from the I signal. The filter is a finite impulse response (FIR) filter having fixed anti-symmetric coefficients and with every other coefficient equal to zero. The I signal is delayed in delay 102 for a period equal to the length of the filter 104.
To compensate for the phase error, the I and Q signals are coupled to a complex multiplier 106 that is driven by an oscillator (e.g., a numerically controlled oscillator (NCO)) 112 that is, in turn, controlled by an error signal from a phase detector 108. The phase of the NCO output signal is modulated to correct the phase error and produce phase adjusted I and Q signals (Ixe2x80x2 and Qxe2x80x2). The phase detector 108 compares the Ixe2x80x2 signal to the derived Qxe2x80x2 signal and produces a phase error detection signal (e) that represents the phase error of the VSB signal. A loop filter 110 (a low pass filter) is used to filter the higher order components from the error signal e. In effect, the combination of the oscillator signal and the I and Q input signals within the complex multiplier 102 creates derotated I and Q signals. The depicted circuit 100 is a closed loop phase tracking circuit that compensates for carrier phase error in the I and Q signals such that a relatively stable in-phase (I) signal is created that can be sampled within a demodulator to extract the data from the VSB signal. One example of a phase tracking circuit that uses a derived Q component is described by Lee et al. in xe2x80x9cA Hardware Efficient Phase/Gain Tracking Loop For The Grand Alliance VSB HDTV Receiver,xe2x80x9d IEEE Trans. on Consumer Electronics, pp. 632-639, Vol. 42, No. 3, Aug. 1996.
Since the Q signal is derived from the I signal to synthesize a complex signal for phase tracking, the derived signal may not always accurately represent the Q signal, i.e., multipath and phase distortion may cause cross-coupling of the I and Q components of the VSB signal such that the derived Q signal misrepresents the actual Q component. When the Q signal is misrepresented, the phase tracking loop will produce an erroneous phase error that will cause the loop to distort the quantized I data. Consequently, the information carried by the I data may not be recoverable.
Therefore, a need exists in the art for a decision directed phase detector that compares the I and Q signals to produce a phase error and uses symbol decision information to enhance the accuracy of the phase error signal.
The disadvantages heretofore associated with the prior art are overcome by a decision directed phase detector that uses the data output from a slicer (quantizer) to aid the phase error detection process to determine a phase error of a complex signal such as a vestigial sideband VSB) signal containing in-phase (I) and quadrature phase (Q) components (I and Q signals). The phase detector comprises a subtractor having one input coupled to the I signal and a second input coupled to the data output of the slicer representing the magnitude of a sample point within a symbol. The quantized signal is subtracted from the I signal to produce xcex94I signal. The Q signal is processed to determine its sign and magnitude. The sign and magnitude values are multiplied together and the result is multiplied with the xcex94I signal to produce the phase error of the VSB signal. This phase error signal is used in a digital television receiver to extract a data signal from a VSB signal for demodulation.